a) Field of the Invention
The present invention relates to a technique for audio data communications, more particularly to a technique for data communications which enables communications of high sound quality audio data.
b) Description of the Related Art
There is a standard for communication among electronic musical instruments called Musical Instrument Digital Interface (MIDI). An electronic musical instrument having a MIDI interface can be connected to other electronic musical instruments via MIDI cables. This connection enables the instruments to communicate MIDI data from each other via the MIDI cables. For example, a player plays a musical piece with one electronic musical instrument and the instrument transmits MIDI data representing the played musical piece to another electronic musical instrument, and the instrument which receives the MIDI data can reproduce the musical piece. That is, connected electronic musical instruments can simultaneously reproduce a musical piece which is played by a player with another instrument.
A communication network connecting a plurality of general purpose computers enables the connected computers to communicate various information with each other. For example, a computer stores information like audio data (representing notes produced by non-electronic musical instruments), MIDI data, or the like in its hard disk drive or the like and transmits the information to other computers via the communication network. The computer which receives the information can store the received information in its storage device such as a hard disk drive. An object of such a general purpose computer communication network is to be a medium for communicating information among computers, however, its property differs from that of the MIDI network which enables the electronic musical instruments to communicate with each other in real time.
The MIDI standard realizes real time communication among the electronic musical instruments, however, it is not suitable for long distance communication and communication among multiple nodes. On the contrary, the general purpose computer network is suitable for the long distance communication and communication among multiple nodes, however, it is not designed for the real time communication among the electronic musical instruments.
The general purpose computer network has been used for multimedia data communications. A case of communicating audio data and other media data (for example, MIDI data) will now be described.
FIG. 2 is a graph showing relationship between communication rate (transfer rate) and a lapse of time between audio data AD and other media data OD. In the graph, the horizontal axis indicates the lapse of time and the vertical axis indicates the transfer rate represented by bit per second (BPS).
The audio data AD is digital data sampled with predetermined sampling frequency. The sound quality of the audio data AD is stable because the audio data AD is sampled with a constant sampling frequency. Therefore, the transfer rate of the audio data AD is also constant as time lapses in principle.
The audio data is often compressed before transmission. In such a case, the transfer rate is not always constant because compression rate may be uneven.
However, such unevenness of the transfer rate is very slight. Regardless of the compression, the audio data is sampled in accordance with a constant sampling frequency per a predetermined period of time, therefore, its sound quality is stable.
The media data OD includes on or a plurality of media data pieces other than the audio data AD, such as MIDI data and/or image data. The MIDI data includes commands such as note-on command indicating a note to be sounded and note-off command indicating a note to be muted. In a case of an electronic keyboard instrument, the note-on command is generated in response to a key-on action and the note-off command is generated in response to a key-off action. The transfer rate of the MIDI data often varies.
Accordingly, the transfer rate of the audio data AD is constant while that of the other media data OD is variable. As for the media data OD, a range of its transfer rate is 0 to B1, for example. B1 represents maximum transfer rate of the media data OD, that is, the total transfer rates of the MIDI data and the image data, for example.
Maximum amount (band width) of data per a predetermined period of time is limited. The total transfer rates of the audio data AD and the other media data OD must not exceed a transfer rate limit B0. Each transfer rate is determined as follows.
Transfer rate B2 of the audio data AD is obtained by the following equation:B2=B0−B1
where B1 represents a maximum transfer rate of the media data OD other than the audio data AD, that is, a peak value in the transfer rate variation, and B0 represents the transfer rate limit.
Each transfer rate is required to be set so that the sum of the transfer rate B2 of the audio data AD and the maximum transfer rate B1 of the other media data OD does not exceed the transfer rate limit B0.
The transfer rate B2 of the audio data AD seldom varies even if the transfer rate of the media data OD varies as time lapses. Since the transfer rate of the media data OD often varies, the total transfer rate of the audio data AD and the media data OD also varies as time lapses. The network occupation rate is sometimes low and sometimes not in accordance with the transfer rate variation. That is, excellent transfer efficiency is not always available.